Li-containing silicon oxide powder and production method thereof

ABSTRACT

There is produced a Li-containing silicon oxide powder containing a crystallized lithium silicate that is mostly water-insoluble Li2Si2O5 and containing little crystalline Si. This object is attained through the mixing of a lower silicon oxide powder represented by a compositional formula SiOx (0.5&lt;x&lt;1.5) with a powdered lithium source that involves grinding of the powdered lithium source; controlling a median diameter D1 of the lower silicon oxide powder and a median diameter D2 of the powdered lithium source so as to fulfill 0.05≤D2/D1≤2; and calcining the mixed powder at 300° C. or higher and 800° C. or lower.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.15/753,038 filed Feb. 15, 2018, which is U.S. Pat. No. 10,427,943,issued on Oct. 1, 2019, and which was the U.S. National Stage ofInternational Application No. PCT/JP2016/071981 filed Jul. 27, 2016,which claims benefit of priority to Japanese Patent Application No.2015-168973 filed Aug. 28, 2015, the entire content of which isincorporated herein by reference.

TECHNICAL FIELD

The present invention concerns silicon oxide powders preferably for usein negative electrode materials of lithium ion secondary batteries andproduction methods thereof, and more specifically Li-containing siliconoxide powders doped with Li and production methods thereof.

BACKGROUND ART

Silicon oxide powders, due to their large electrical capacity, are knownto serve as superior negative electrode materials for lithium ionsecondary batteries. And yet, the silicon oxide powders serving as thenegative electrode materials for lithium ion secondary batteries are lowin initial efficiency. A method known to address this problem is lithiumdoping (Li-doping). Specifically, Li-doping of the silicon oxide powderis intended to improve the initial efficiency by allowing initialcharging to be done with the inhibition the formation of lithiumcompounds that do not contribute to the charging-discharging.

The Li-doping of the silicon oxide powder forms lithium silicate.Lithium silicate has a phase varying depending on an amount of theLi-doping. Specifically, with an increased amount of the Li-doping,lithium silicate has a phase varying from Li₂Si₂O₅, through Li₂SiO₃, toLi₄SiO₄, as shown in chemical formulae below.SiO₂+⅖Li→⅕Li₂Si₂O₅+⅗SiSiO₂+⅔Li→⅓Li₂SiO₃+⅔SiSiO+Li→¼Li₂SiO₄+¾Si  [Chem. 1]

Lithium silicate, at its amorphous state, is reactive with water, butthrough its crystallization, particularly in the case of Li₂Si₂O₅, comesto be unreactive with water. This is favorable from viewpoints such asits handlability. It is therefore desirable that Li-containing siliconoxide powder having undergone the Li-doping have lithium silicate mostlyin the form of a crystallized Li₂Si₂O₅. Meanwhile, Li₂SiO₃ and Li₄SiO₄in lithium silicate, even through crystallization, remain reactive withwater, which is a reason for such problems as lithium elution during itshandling process.

The crystallization process of lithium silicate in the Li-containingsilicon oxide powder is accompanied by Si advancing towardcrystallization. Crystallization of Si would have an adverse effect oncycle properties of batteries. While the amorphous Si in the absence ofheat can retain its amorphous state by, for example, an electrochemicalLi-doping, the electrochemical Li-doping however would permit thelithium silicate, too, to be amorphous, and would involve the difficultyin obtaining powders.

It is desirable in view of the above circumstances that Li-containingsilicon oxide powders for use in negative electrode materials forlithium ion secondary batteries be the ones that have a crystallizedlithium silicate, in particular, crystallized Li₂Si₂O₅, and have anamorphous Si.

Such Li-containing silicon oxide powders are produced, for example, by apowder calcining-based method that includes mixing the silicon oxidepowder with a powdered lithium source, and calcining the resultantmixture (Patent Literatures 1 to 4). The silicon oxide powders givenhere are produced by heating a mixture of silicon dioxide and silicon toform a silicon monoxide gas, cooling the resultant silicon monoxide gasfor precipitation, and finely grinding the precipitate. The siliconoxide powders produced by such a precipitation method are mostlyamorphous enough to have a lowered thermal expansion coefficient, andtherefore are said to be advantageous from such viewpoints asaccomplishing improved cycle properties.

However, such powder calcining-based Li-doping methods have a problem:lithium silicate formed by such methods includes not justwater-insoluble Li₂Si₂O₅ but also water-soluble Li₂SiO₃ andwater-soluble Li₄SiO₄. This problem has been addressed by avoidingadding an increased Li-doping amount at the time of Li-doping.Nonetheless, the powders having undergone the Li-doping in a decreasedamount represented by, for example, Li/O (atomic ratio)=0.2, whichamount is considered from an equilibrium standpoint to contribute to theinhibition of the formation of Li₂SiO₃, in fact, exhibit peaks ofLi₂SiO₃ in the observation by XRD measurement: the presence of thisproblem has been identified by experiments made by the inventors of thepresent invention.

Further decreasing the Li-doping amount would limit the formation oflithium silicate itself, depriving the Li-doping of its significance,which would make it impossible to hope for the enhancement of theinitial efficiency intended by the Li-doping. Excessively increasing theLi-doping amount, in contrast, (for example, Li/O (atomic ratio)≥1),would cause the lithium silicate phase to be occupied by Li₄SiO₄ alonewith Li₂SiO₃ absent, which is a reason for the exhibition of such anexcessive activity as to make its usability inferior.

The inventors have found that the powder calcining-based Li-doping,because of involving heating, could involve disproportionation ofsilicon oxide that will produce the crystalline Si and that tends tooccur at a lower temperature. Another tendency is that an increaseddoping amount in the Li-doping would lead to an increased amount ofmetal Si, as is clear from the chemical formulae set forth hereinabove.

In view of the above, the applicant's corporation has performed thepowder calcining-based Li-doping at a decreased Li-doping amount and ata lower thermal treatment temperature. And yet, even at a heat treatmenttemperature at which the disproportionation of silicon oxide shouldintrinsically not take place (for example, not higher than 700° C.), theresultant powder undergoes the disproportionation of silicon oxide and,in observation by powder XRD measurement, exhibits peaks including ahigh peak attributed to the crystalline Si, which is a risk for reducedcycle properties.

To the knowledge of the applicant in view of the foregoing,Li-containing silicon oxide powders containing a crystallized lithiumsilicate that is mostly water-insoluble Li₂Si₂O₅ and containing littlecrystalline Si are nonexistent.

Another approach, aside from the Li-doping, taken to enhance the cycleproperties is allowing particles (powder particles) constituting siliconoxide to undergo a carbon coating treatment (C-coating). In PatentLiterature 3, Li-doping is followed by C-coating; and in PatentLiterature 4, C-coating is followed by Li-doping.

CITATION LIST Patent Literature

Patent Literature 1: JP-B-2997741

Patent Literature 2: JP-B4702510

Patent Literature 3: JP-B4985949

Patent Literature 4: JP-A-2011-222153

SUMMARY OF THE INVENTION Technical Problem

An object of the present invention is provided a Li-containing siliconoxide powder containing a crystallized lithium silicate that is mostlywater-insoluble Li₂Si₂O₅ and containing little crystalline Si; and aproduction method thereof.

Solution to Problem

To attain the object described above, the present inventors haveextensively studied why performing the powder calcining-based Li-dopingin a decreased amount considered from an equilibrium standpoint tocontribute to the inhibition of the formation of Li₂SiO₃ still let theformation of Li₂SiO₃. Their studies have revealed facts described below.

The formation of Li₂SiO₃ in the Li-doping even in a decreased Li-dopingamount considered from an equilibrium standpoint to contribute to theinhibition of the formation of Li₂SiO₃, is presumed to mean that theprocess of calcining the silicon oxide powder and the powdered lithiumsource involves a non-uniform reaction taking place on particlesconstituting the silicon oxide powder (powder particles), and that thesurface or the inside of the particles locally has the concentration ofLi.

A possible reason therefor is an excessively large particle diameter ofthe powdered lithium source as compared with the particle diameter ofthe silicon oxide powder. For an instance, the particle diameter of thesilicon oxide powder in Li-doping, when expressed as a median diameter,ranges from 5 to 10 μm, whereas the particle diameter of the powderedlithium source, when expressed as a median diameter, is about 20 μm. Itis believed that performing the Li-doping step using the powderedlithium source having such a larger particle diameter would cause, asshown in FIG. 1(a), particles 1 of the silicon oxide powder to belocally contacted by particles 2 of the powdered lithium source, inwhich the local reaction occurring at the contacting sites, which isaccompanied by the increase in the reaction temperature, results in theconcentration of lithium.

In view of the above, mixing the silicon oxide powder and the powderedlithium source and calcining the mixture were preceded byfinely-grinding the powdered lithium source. The resultant silicon oxidepowder having undergone the Li-doping in this way, when subjected topowder XRD measurement, exhibits peaks including a lowered peakattributed to Li₂SiO₃ together with a lowered peak attributed to thecrystalline Si. It can be assumed from this result that reducing theparticle diameter of the powdered lithium source as compared with theparticle diameter of the silicon oxide powder will allow the Li-dopingstep to be done with the periphery of particles 1 of the silicon oxidepowder thinly coated by particles 2 of the powdered lithium source, asillustrated in FIG. 1(b). This will allow the particles 1 of the siliconoxide powder to undergo the reaction in a uniform manner that suppressthe local reaction and results in inhibiting the local concentration oflithium.

The Li-containing silicon oxide powder of the present invention, whichis founded on such knowledge as described above, is for use in anegative electrode material of a lithium ion secondary battery, with theLi-containing silicon oxide powder undergoing Li-doping, and such aLi-containing silicon oxide powder exhibits peaks observed by X-raydiffractometry using CuKα ray that include a height of peak attributedto Li₂Si₂O₅, P1, exhibited at a diffraction angle 2θ ranging from 24.4to 25.0°; a height of peak attributed to Li₂SiO₃, P2, exhibited at adiffraction angle 2θ ranging from 18.6 to 19.2°; and a height of peakattributed to crystalline Si, P3, exhibited at a diffraction angle 2θranging from 27.4 to 29.4°, which fulfill Requirement (1): P2/P1<1 andP3/P1<0.5.

The method according to the present invention for producing theLi-containing silicon oxide powder is a method for producing aLi-containing silicon oxide powder for use in a negative electrodematerial of a lithium ion secondary battery, the method comprising astep of mixing a lower silicon oxide powder represented by acompositional formula SiO_(x) (0.5<x<1.5) with a powdered lithiumsource, and a step of calcining the mixed powder at 300° C. or higherand 800° C. or lower, wherein a median diameter D1 of the lower siliconoxide powder and a median diameter D2 of the powdered lithium sourcefulfill Requirement (2): 0.05≤D2/D1≤2.

In the Li-containing silicon oxide powder of the present invention, theheight of peak attributed to Li₂Si₂O₅, P1, is given by subtracting abackground intensity from that peak intensity, in which the peakintensity is a maximum diffraction intensity at a diffraction angle 2θranging from 24.4 to 25.0°, and the background intensity is an intensityat 2θ=24.7° on a straight line connecting a diffraction intensity at adiffraction angle 2θ of 24.4° and a diffraction intensity at adiffraction angle 2θ of 25.0°.

Likewise, a height of peak attributed to Li₂SiO₃, P2, exhibited at adiffraction angle 2θ ranging from 18.6 to 19.2°, is given by subtractinga background intensity from that peak intensity, in which the peakintensity is a maximum diffraction intensity at a diffraction angle 2θranging from 18.6 to 19.2°, and the background intensity is an intensityat 2θ=19.9° on a straight line connecting a diffraction intensity at adiffraction angle 2θ of 18.6° and a diffraction intensity at adiffraction angle 2θ of 19.2°.

Likewise, a height of peak attributed to crystalline Si, P3, exhibitedat a diffraction angle 2θ ranging from 27.4 to 29.4°, is given bysubtracting a background intensity from that peak intensity, in whichthe peak intensity is a maximum diffraction intensity at a diffractionangle 2θ ranging from 27.4 to 29.4°, and the background intensity is anintensity at 2θ=28.4° on a straight line connecting a diffractionintensity at a diffraction angle 2θ of 27.4° and a diffraction intensityat a diffraction angle 2θ of 29.4°.

In any cases of the determination of peak intensities, the calculationof the peak intensities can employ data that are obtained by X-raydiffractometry using CuKα ray and prepared at a diffraction angleinterval of 0.02° and converted, with the number of data specified of11, to moving average trendlines. The use of the moving averagetrendlines reduces measurement errors caused by the fluctuation ofdiffraction intensities.

The above-identified object is attained by the Li-containing siliconoxide powder of the present invention in which the heights of the peaks,P1, P2 and P3, fulfill Requirement (1): P2/P1<1 and P3/P1<0.5. The ratioP2/P1 of 1 or more, meaning a more amount of Li₂SiO₃ produced in theLi-doping, would lead to problems including the elution of lithiumresulting in pH increase in a slurrying step, which in turn leads toinferior binder properties and reduced initial efficiency. The P2/P1,which is preferably smaller, is desirably 0.8 or less, and particularlydesirably 0.3 or less. The P3/P1 is 0.5 or more, meaning a more amountof the crystalline Si produced in the Li-doping step, would lead to theexhibition of the inferior cycle properties. The P3/P1, which isfavorably smaller, is preferably 0.3 or less, and particularlypreferably 0.1 or less.

It is preferable that the Li-doping amount (Li content) in theLi-containing silicon oxide powder of the present invention bedetermined in view of the optimization of an amount of Li₂Si₂O₅ inlithium silicate so as to fulfill 0.2≤Li/O≤0.6 in terms of elementratio. The ratio Li/O≤0.2 would mean insufficiency of lithium silicateitself. The ratio Li/O>0.6, too, would mean insufficiency of Li₂Si₂O₅.Namely, the ratio Li/O>0.67 is considered from an equilibrium standpointnot to contribute to the formation of Li₂Si₂O₅; and the ratio Li/O≤0.67but with an increased amount of Li would permit the local reaction toreadily proceed, which would be accompanied by a reaction with bindersin assembling a battery, the resultant battery failing to havesufficient performance.

In the Li-containing silicon oxide powder of the present invention,particles constituting the powder (i.e., powder particles) preferablyhave a particle diameter, expressed in terms of a median diameter,ranging from 0.5 to 30 μm. The median diameter, D₅₀, refers to aparticle diameter present at cumulative 50% from the finer-particle side(or more-coarse particle side) in the volume-based cumulativeparticle-size distribution, and can be measured by using a laserdiffraction-type particle size distribution measuring equipment. Byhaving the median diameter ranging from 0.5 to 30 μm, the powder attainsgood dispersibility, and in its use in the form of a slurry in anegative electrode of a lithium ion secondary battery, allows the slurryto have a viscosity suitable for its application. Another benefitaccomplished in the use in the negative electrode of the lithium ionsecondary battery is successful suppression of an electrolyte solutionfrom reacting with surfaces of the particles constituting the negativeelectrode thereby inhibiting the battery life from shortening. Themedian diameter especially desirable ranges 0.5 to 15 μm.

In another preferable embodiment of the Li-containing silicon oxidepowder of the present invention, at least part of the particlesconstituting the powder (i.e., powder particles) has a conductive carbonfilm. The formation of the conductive carbon film improves theelectrical conductivity among powder particles constituting the negativeelectrode, and the electrical conductivity between the negativeelectrode and its base, i.e., a collector, resulting in the provision ofthe lithium secondary battery with improved cycle properties. Theformation of the conductive carbon film herein will be referred to asC-coating.

The amount of the conductive carbon film possessed by the particlesconstituting the Li-containing silicon oxide powder (i.e., powderparticles), when expressed in terms of a weight proportion of carbonwith respect to a mass of the whole of the silicon oxide powder,preferably ranges from 0.5 to 20 wt %. The formation of the film in anamount less than 0.5 wt % would weaken the significance of the formationof the conductive carbon film with respect to the powder particles. Onthe other hand, the formation of the film in an amount more than 20 wt%, meaning a lowered ratio of the silicon oxide powder relative to thewhole of the active material, would weaken the capacity increasebenefits intended by the use of the silicon oxide powder. The amount ofthe film formed is especially preferably from 0.5 to 7 wt %, and morepreferably from 0.5 to 5 wt %.

In the method according to the present invention for producing theLi-containing silicon oxide powder, the silicon oxide powder to besubjected to the mixing step (i.e., silicon oxide powder raw material)is a lower silicon oxide powder represented by a compositional formulaSiO_(x) (0.5<x<1.5) for reasons that the ratio x≤0.5 would cause thelithium ion secondary battery to have inferior cycle properties and theratio x≥1.5 would give rise to the reduction in the initial efficiencyand in the capacity. The compositional formula desirable in particularis 0.7<x<1.3.

Examples of the powdered lithium source to be mixed with the siliconoxide powder in the mixing step include lithium hydride, lithium oxide,lithium hydroxide and lithium carbonate. Desirable in particular amongthose is lithium hydride, the use of which is accompanied by littleamount of by-products and infrequent reduction of battery capacity.

The mixing ratio between the silicon oxide powder and the powderedlithium source that is preferable considering, in the calcining stepfollowing the mixing step, the promotion of the formation ofwater-insoluble Li₂Si₂O₅, the inhibition of the formation ofwater-soluble Li₂SiO₃, and the inhibition of the formation of thecrystalline Si, when expressed in terms of an element ratio, moreaccurately, an element ratio between 0 in the lower silicon oxide powderand Li in the powdered lithium source, is 0.2≤Li/O<0.6. The ratioLi/O>0.67 is considered from an equilibrium standpoint not to contributeto the formation of Li₂Si₂O₅. The ratio Li/O≤0.67 but with an increasedamount of Li would permit the local reaction to readily proceed, whichwould be accompanied by a reaction with binders in assembling a battery,the resultant battery failing to have sufficient performance.

From an equilibrium standpoint, with Li/O being less than 0.4, Li₂Si₂O₅alone is formed; with Li/O reaching 0.4, Li₂SiO₃ begins to be formed;and with Li/O reaching 0.67, Li₂Si₂O₅ stops being formed and insteadLi₄SiO₄ begins to be formed (see the chemical formula 1).

Of particular significance in the method according to the presentinvention for producing the Li-containing silicon oxide powder iscontrolling the ratio between the median diameter D1 of the lowersilicon oxide powder serving as the silicon oxide powder raw materialand the median diameter D2 of the powdered lithium source to be mixedtherewith (median diameter ratio D2/D1) to fall within the range from0.05 to 2. This is in view of frequent cases where the median diameterof the silicon oxide powder for use in a negative electrode material ofa lithium ion secondary battery ranges from 0.5 μm to 30 μm: atD2/D1<0.05, the powdered lithium source excessively small would changeitself to secondary particles, failing to give the benefits of theuniform mixing, and at D2/D1>2, the powdered lithium source excessivelylarger than the silicon oxide powder would permit the localconcentration of lithium to readily occur at the time of the reaction.The ratio D2/D1 preferable considering the inhibition of theconcentration of lithium at the time of the reaction is from 0.05 to 1,and particularly preferably from 0.05 to 0.5. As described above, theinhibition of the concentration of lithium at the time of the reactionof the silicon oxide powder leads to the inhibition of the formation ofLi₂SiO₃ together with the inhibition of the formation of the crystallineSi.

The median diameter ratio D2/D1 ranging from 0.05 to 2 that is given bythe silicon oxide powder raw material and the powdered lithium source tobe mixed therewith is controlled by grinding the powdered lithium sourcebefore its mixing with the silicon oxide powder or after its mixing withthe silicon oxide powder. The grinding may be done, for example, bymanually grinding the powdered lithium source with a mortar followed bysubjecting the powder thus ground to a sieve with a small opening, or amethod using a ball mill or a bead mill. The use of a grinding mediumhaving a Mohs' hardness that is lower than the silicon oxide powder andhigher than the powdered lithium source improves productivity, sinceusing such a medium for mixing the silicon oxide powder with thepowdered lithium source that is followed by grinding the mixture wouldenable a selective grinding of the powdered lithium sourcesimultaneously with the mixing.

The calcining temperature in the calcining step is controlled to rangefrom 300° C. to 800° C.: the calcining at an excessively highertemperature would invite the disproportionation of silicon oxide therebycausing the precipitation of the crystalline Si and leading to theexhibition of inferior cycle properties; and the calcining at anexcessively lower temperature would make it difficult for the Li-dopingreaction to proceed thereby leading to such disadvantages asinsufficiency of lithium silicate and excessively prolonged reactiontime. The calcining temperature preferably ranges from 300° C. to 700°C. The calcining temperature is particularly preferably from 400° C. to700° C., and more preferably from 500° C. to 650° C. The calciningatmosphere is preferably an inert gas atmosphere, in particular, anargon gas atmosphere.

In the method according to the present invention for producing theLi-containing silicon oxide powder, the silicon oxide powder to besubjected to the mixing step preferably undergoes a carbon coatingtreatment (C-coating) to have a conductive carbon film. The C-coatingmay be a known thermal treatment, for example, a thermal CVD methodusing a hydrocarbon gas.

Such a thermal treatment, which provides at least part of the particlesconstituting the silicon oxide powder (i.e., powder particles) with theconductive carbon film, improves the electrical conductivity amongpowder particles constituting the negative electrode, and the electricalconductivity between the negative electrode and its base, i.e., acollector, resulting in the provision of the lithium secondary batterywith improved cycle properties.

In particular, allowing the silicon oxide powder which will be latersubjected to the mixing step to undergo the C-coating (i.e., allowingthe silicon oxide powder to undergo the C-coating prior to theLi-doping) would lead to avoiding adverse effects caused by the decreaseof the temperature involving the disproportionation in the Li-doping.Specifically, considering frequent cases where the C-coating temperatureis higher than the Li-doping temperature as well as the tendency of thetemperature involving the disproportionation being decreased by theLi-doping, great concern would arise, in the case of performing theLi-doping followed by the C-coating, that the C-coating will advance thepossible disproportionation. This concern is eliminated in the case ofperforming the C-coating followed by the Li-doping, and in this way theformation of the crystalline Si caused by the possibledisproportionation is inhibited.

In addition, performing the C-coating followed by the Li-doping allowsthe carbon film of the particles constituting the silicon oxide powderto be interposed between those particles and the particles constitutingthe particulate lithium source, and thereby allowing the film to serveas a buffer layer (buffer). The C-coating, too, is therefore expected toinhibit the local concentration of lithium, which in turn inhibits theformation of Li₂SiO₃ and the crystalline Si.

The method for producing the Li-containing silicon oxide powder of thepresent invention is not limited to the above-described powder calciningmethod that includes mixing the above-described silicon oxide powderwith the powdered lithium source and calcining the mixture, and istherefore applicable to other thermal Li-doping methods involvingheating as well as to other methods than the thermal Li-doping methods:specifically an electrochemically Li-doping followed by performing athermal treatment at an appropriate temperature.

Advantageous Effects of the Invention

The Li-containing silicon oxide powder of the present invention containsa crystallized lithium silicate that is mostly water-insoluble Li₂Si₂O₅and contains little water-soluble Li₂SiO₃. This configuration inhibitsthe elution of lithium leading to pH increase in a process to providethin film electrodes, which in turn suppresses inferior binderproperties and reduced initial efficiency. The inclusion of littleamount of the crystalline Si contributes to inhibiting the cycleproperties from being lowered by the crystalline Si.

The method according to the present invention for producing theLi-containing silicon oxide powder, which is capable of producing aLi-containing silicon oxide powder containing a crystallized lithiumsilicate that is mostly water-insoluble Li₂Si₂O₅ and containing littlewater-soluble Li₂SiO₃, is effective in preventing the elution of lithiuminviting an increased pH increase in a slurrying step, which in turnsuppresses inferior binder properties and reduced initial efficiency.Also, the method according to the present invention, which enables thesilicon oxide powder to be produced with the inhibition of the formationof the crystalline Si, is effective in preventing the reduction of thecycle properties caused by the formation of the crystalline Si.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 schematically shows powder particles, shown in (a), producedaccording to conventional methods, which are compared with powderparticles, shown in (b), produced according to the method of the presentinvention for producing a Li-containing silicon oxide powder.

FIG. 2 shows an X-ray diffraction chart of a Li-containing silicon oxidepowder produced according to the present invention.

FIG. 3 shows an X-ray diffraction chart of a conventional Li-containingsilicon oxide powder.

FIG. 4 shows an X-ray diffraction chart of silicon oxide powder notdoped with Li.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described hereinafter. Themethod according to embodiments of the present invention for producing aLi-containing silicon oxide powder begins with the preparation of asilicon oxide powder serving as a raw material used in the productionmethod, and of a powdered lithium source to be mixed therewith.

The silicon oxide powder serving as a raw material is a lower siliconoxide powder represented by a compositional formula SiO_(x) (0.5<x<1.5).Used here is an amorphous SiO produced by precipitation method, i.e.,the one represented by SiO_(x) (x=1). The silicon oxide powder has aparticle diameter, in terms of median diameter, of 0.5 to 30 μm.

Examples of the powdered lithium source include lithium hydride (LiH),lithium oxide (Li₂O), lithium hydroxide (LiOH) and lithium carbonate(Li₂CO₃). Used here is lithium hydride (LiH).

The silicon oxide powder serving as the raw material is subjected toC-coating that gives a carbon film. The C-coating is performed by athermal CVD method using a hydrocarbon gas, for example, under anatmosphere of a mixed gas of propane and argon, at 850° C. The C-coatingamount when expressed in terms of a weight proportion of carbon withrespect to a mass of the whole of the silicon oxide powder is 0.5 to 20wt %.

The powdered lithium source is subjected to grinding treatment. Thegrinding treatment is done, for example by the use of a mortar, so as togive a controlled particle size after grinding such that the powderedlithium source and the silicon oxide powder provide the median diameterratio (D2/D1) ranging from 0.05 to 2, preferably from 0.05 to 1.

Subsequently, the C-coated silicon oxide powder is mixed with thegrinding-treated powdered lithium source. The mixing ratio is controlledto give a Li/O molar ratio ranging from 0.2 to 0.6 considering, from anequilibrium standpoint, the promotion of the formation of Li₂Si₂O₅together with the inhibition of the formation of Li₂SiO₃.

Subsequently, the mixed powder of the silicon oxide powder and thepowdered lithium source is subjected to its calcining under an inert gasatmosphere. The calcining temperature ranges from 300 to 800° C., andmore specifically such a temperature region as to avoid involving thedisproportionation of the silicon oxide powder. The calcining allows thesilicon oxide powder to undergo the Li-doping, and consequently givesthe Li-containing silicon oxide powder.

Here, the mixing ratio of the powdered lithium source to the siliconoxide powder restricted to be in the lower range considered from anequilibrium standpoint to contribute to the inhibition of the formationof Li₂SiO₃, and the powdered lithium source finely ground so that boththe powders give the median diameter ratio (D2/D1) ranging from 0.05 to2, particularly from 0.05 to 1, lead to the inhibition of the localreaction on the particles constituting the silicon oxide powder therebyinhibiting the concentration of lithium, with a result that the lithiumsilicate phase of the Li-containing silicon oxide is mostly composed ofLi₂Si₂O₅, and concurrently the formation of the crystalline Si in theLi-containing silicon oxide is inhibited.

Specifically, the produced Li-containing silicon oxide powder, whenobserved by X-ray diffractometry using CuKα ray, exhibits peaks thatinclude a height of peak attributed to Li₂Si₂O₅, P1, exhibited at adiffraction angle 2θ ranging from 24.4 to 25.0°; a height of peakattributed to Li₂SiO₃, P2, exhibited at a diffraction angle 2θ rangingfrom 18.6 to 19.2°; and a height of peak attributed to crystalline Si,P3, exhibited at a diffraction angle 2θ ranging from 27.4 to 29.4°,fulfill P2/P1<1 and P3/P1<0.5.

The produced Li-containing silicon oxide powder is used as a negativeelectrode material of a lithium ion secondary battery. Specifically, theLi-containing silicon oxide powder is mixed with an aqueous binder togive a slurry, and the slurry is applied on a collector composed ofe.g., a copper foil, followed by drying, to provide a thin film workingelectrode. The lithium silicate phase in the Li-containing silicon oxidepowder is mostly composed of water-insoluble Li₂Si₂O₅ and hardlycontains water-soluble Li₂SiO₃. This configuration prevents the elutionof lithium from the silicon oxide, and improves the initial efficiencyincluded in battery performances as intended. Also, the configurationinhibits the silicon oxide from having the crystalline Si, andaccordingly inhibits the reduction of the cycle properties included inbattery performances. Improvement benefits attained by the inhibition ofthe formation of Li₂SiO₃, which is Li-rich, are seen not just in the useof an aqueous binder but also in the use of a solvent-based bindercomposed of an organic solvent as a solvent, since most of such polymercomponents as polyimides is reactive with lithium.

EXAMPLES Example 1

A silicon oxide powder serving as a raw material prepared for producinga Li-containing silicon oxide powder was an amorphous SiO powderproduced by precipitation method. The SiO powder raw material had amedian diameter of 8.0 μm. For the SiO powder raw material to undergothe C-coating through a thermal treatment, the SiO powder wasthermally-treated at 850° C. for 30 minutes in a furnace into which acarbonizing gas was supplied at a flow rate of 1 liter per minute, thecarbonizing gas being given by mixing argon and propane at a weightratio of 1:1.

Subjecting the C-coated SiO powder to combustion-infrared absorptionmethod revealed that the particles constituting the Si powder had aconductive carbon film formed at a weight ratio of 1.00%. The C-coatedSiO powder had a median diameter D1 of 8.2 μm.

A powdered lithium source selected for mixing with the raw material SiOwas LiH powder, which originally had a median diameter of 20.8 μm,considerably larger than the median diameter of the C-coated SiO powder.The LiH powder was thus finely ground with a mortar in a glove box underan argon atmosphere, and the ground powder was classified by using atest sieve with an opening of 16 μm.

The finely-ground LiH powder was subjected to a dry-particle sizedistribution measurement using a laser diffraction-type particle sizedistribution measuring equipment, HELOS, manufactured by Sympatec GmbH.The finely-ground LiH powder was found to have a median diameter D2 of5.1 μm, which was smaller than the median diameter D1 of the C-coatedSiO powder (8.2 μm), which gave a median diameter ratio D2/D1 of 0.62.

For Li-doping treatment, the C-coated SiO powder (median diameter 8.2μm) was mixed with the finely-ground LiH powder so as to give a Li/Omolar ratio of 0.5, and thereafter the mixture was calcined in an argonatmosphere in a furnace (1 atm, 600° C.) for 72 hours.

In the X-ray diffractometry using CuKα ray of the produced Li-containingSiO powder, a height of peak attributed to Li₂Si₂O₅, P1, exhibited at adiffraction angle 2θ ranging from 24.4 to 25.0°; a height of peakattributed to Li₂SiO₃, P2, exhibited at a diffraction angle 2θ rangingfrom 18.6 to 19.2°; and a height of peak attributed to crystalline Si,P3, exhibited at a diffraction angle 2θ ranging from 27.4 to 29.4° wereexamined to calculate P2/P1 and P3/P1.

The SiO powder undergoing the C-coating and the Li-doping was used tofabricate a negative electrode of a lithium ion secondary battery.Specifically, the SiO powder, ketjen black, and a polyimide precursorserving as a non-aqueous solvent-based binder were mixed together at amass ratio of 85:5:10. To the mixture, NMP (n-methylpyrrolidone) wasadded, followed by kneading, to give a slurry. The slurry was applied ona copper foil with a thickness of 40 μm, and pre-dried at 80° C. for 15minutes. This was followed by punching so as to give a diameter of 11mm, and further by an imidizing treatment. In this way, a negativeelectrode was provided.

The fabricated negative electrode was used to produce a lithium ionsecondary battery. A counter electrode used was a lithium foil. Anelectrolyte used was a solution prepared by dissolving LiPF₆ (lithiumhexafluorophosphate) at 1 mol/L in a solution that had been given bymixing ethylene carbonate with diethyl carbonate at a volume ratio of1:1. A separator used was a polyethylene porous film with a thickness of30 μm. In this way, a coin cell was fabricated.

The fabricated lithium ion secondary battery was subjected tocharging-discharging tests with the use of a secondary batterycharging-discharging tester (manufactured by NAGANO Co., Ltd.).Conditions for the charging-discharging operations are shown in Table 1.Through the charging-discharging tests, initial charging capacity,initial discharging capacity, ratio of the initial discharging capacityto the initial charging capacity (hereinafter referred to as “initialefficiency”), and ratio of discharging capacity given at 50th cyclesrelative to the initial discharging capacity (hereinafter referred to asthe “discharging capacity retentivity after 50 cycles”) were determined.

TABLE 1 Charging Discharging 1st CC-CV 0.1 C 5 mV-0.01 C CC 0.1 C 1.5 V2nd CC-CV 0.3 C 5 mV-0.01 C CC 0.3 C 1.5 V 3rd~50th CC-CV 0.5 C 5mV-0.01 C CC 0.5 C 1.5 V

Example 2

The same operation as in Example 1 was performed, except that in Example1, the SiO powder raw material was changed to the one with a mediandiameter of 5.6 μm, which was smaller than in Example 1, and togetherwith this change, the thermal treatment time for the C-coating wasshortened from 30 minutes to 27 minutes. The C-coating amount in theC-coated SiO powder was 0.94 wt %. The median diameter D1 was 5.8 μm,and the median diameter ratio D2/D1 was 0.88.

Example 3

The same operation as in Example 1 was performed, except that in Example1, the Li-doping was performed so that the mixing ratio between theC-coated SiO powder and the finely-ground LiH powder (Li/O molar ratio)was 0.2.

Example 4

The same operation as in Example 2 was performed, except that in Example2, the Li-doping was performed so that the mixing ratio between theC-coated SiO powder and the finely-ground LiH powder (Li/O molar ratio)was 0.2.

Comparative Example 1

The same operation as in Example 2 was performed, except that in Example2, the Li-doping of the SiO powder that had been C-coated (mediandiameter 8.2 μm) involved the use of a LiH powder that did not undergothe fine grinding and had a median diameter of 20.8 μm. The mediandiameter ratio D2/D1 was 2.54.

Comparative Example 2

The same operation as in Example 4 was performed, except that in Example4, the Li-doping of the SiO powder that had been C-coated (mediandiameter 8.2 μm) involved the use of a LiH powder that did not undergothe fine grinding and had a median diameter of 20.8 μm.

Results of X-ray diffractometry of Li-containing SiO powders produced inExamples 1 to 4 and Comparative Examples 1 and 2 (P2/P1 and P3/P1), andresults of charging-discharging tests (initial charging capacity,initial discharging capacity, initial efficiency, and dischargingcapacity retentivity after 50 cycles), together with mixing ratiosbetween SiO powder and LiH powder (Li/O molar ratio) in Li-doping andmedian diameter ratios D2/D1, are shown in Table 2.

An X-ray diffraction chart of the Li-containing SiO powder produced inExample 2 is shown in FIG. 2, and an X-ray diffraction chart of theLi-containing SiO powder produced in Comparative Example 1 is shown inFIG. 3. An X-ray diffraction chart of a SiO powder not doped with Li isshown in FIG. 4 for reference purpose.

TABLE 2 Discharging capacity Initial Initial retentivity chargingdischarging Initial after 50 Li/O D2/D1 P2/P1 P3/P1 capacity capacityefficiency cycles Example 1 0.5 0.62 0.21 0.07 1853 1568 84.6 86.5Example 2 0.5 0.88 0.71 0.20 1802 1515 84.1 87.2 Example 3 0.2 0.62 0.050.05 2050 1615 78.8 87.3 Example 4 0.2 0.88 0.11 0.10 2203 1679 76.288.5 Comparative 0.5 2.54 1.27 0.79 1900 1225 64.5 2.1 Example 1Comparative 0.2 2.54 1.12 1.78 2281 1710 75.0 53.1 Example 2

SiO powder not doped with Li does not exhibit any crystalline peaks, asis evident from FIG. 4, and is therefore practically amorphous. TheX-ray diffraction chart shown in FIG. 4 is of the SiO powder not dopedwith Li and not C-coated, which SiO powder was found to be the one thateven after undergoing the C-coating at C-coating temperature of 850° C.did not exhibit any crystalline peaks. Allowing this SiO powder toundergo the C-coating and thereafter Li-doping produced powders, inComparative Examples 1 to 2, that in spite of having undergone theLi-doping at a low temperature of 600° C. markedly exhibited not justpeaks attributed to Li₂Si₂O₅ but also peaks attributed to Li₂SiO₃ andthe crystalline Si, as is evident from FIG. 3.

The above result is contrasted with Examples 1 to 4, where allowing SiOpowder to undergo the C-coating and thereafter the Li-doping producedpowders that exhibited high peaks attributed to Li₂Si₂O₅ but with lowpeaks attributed to Li₂SiO₃ and to the crystalline Si, as is evidentfrom FIG. 2. This is due to the fact that the Li-doping was preceded byfinely grinding the LiH powder serving as a raw material of theLi-doping to give a median diameter ratio D2/D1 that had been kept loweras compared with the SiO powder, which accordingly inhibited the localreaction from occurring on the SiO powder particles and suppressed theconcentration of lithium, resulting in the inhibition of the formationof Li₂SiO₃ and the formation of the crystalline Si.

In fact, as is evident from Table 2, Examples 1 to 4, as compared withComparatives Examples 1 and 2, the ratio P2/P1 of a height of peakattributed to Li₂SiO₃, P2, relative to a height of peak attributed toLi₂Si₂O₅, P1, is kept to be not higher than 1; and moreover the ratioP3/P1 of a height of peak attributed to crystalline Si, P3, relative toa height of peak attributed to Li₂Si₂O₅, P1, is kept to as low as 0.This resulted in superior battery performances: high initial efficiencyvalues and high discharging capacity retentivity values after 50 cycles.

The comparison between Examples 1 versus Example 3 and the comparisonbetween Example 2 versus Example 4 each show that the former exampleattained a larger P2/P1 than the latter example, i.e., the formerexamples having a higher initial efficiency: this is due to a moreamount of Li-doping amount (Li/O) in the Li-doping. The comparisonbetween Example 1 versus Example 2 and the comparison between Example 3versus Example 4 each show that the latter example attained a lagerP2/P1 than the former example: this will be due to a larger mediandiameter ratio D2/D1 of the LiH powder to the SiO powder.

The invention claimed is:
 1. A method for producing a Li-containingsilicon oxide powder for use in a negative electrode material of alithium ion secondary battery, the method comprising: a step of mixing alower silicon oxide powder represented by a compositional formulaSiO_(x) (0.5<x<1.5) with a powdered lithium source, and a step ofcalcining the mixed powder at 300° C. or higher and 800° C. or lower,wherein a median diameter D1 of the lower silicon oxide powder and amedian diameter D2 of the powdered lithium source fulfill Requirement(2): 0.05≤D2/D1≤2.
 2. The method for producing a Li-containing siliconoxide powder according to claim 1, comprising grinding the powderedlithium source to be mixed with the lower silicon oxide powder for thefulfillment of Requirement (2).
 3. The method for producing aLi-containing silicon oxide powder according to claim 1, wherein amixing ratio of the powdered lithium source to the lower silicon oxidepowder at the mixing step is 0.2≤Li/O≤0.6, when the mixing ratio isexpressed in terms of an elemental ratio Li in the powdered lithiumsource relative to 0 in the lower silicon oxide powder.
 4. The methodfor producing a Li-containing silicon oxide powder according to claim 1,comprising allowing the silicon oxide powder to be subjected to themixing step to undergo a carbon coating treatment that gives aconductive carbon film.